This invention relates to methods for the preparation of filter media and more particularly, to the provision of anionically charged media of enhanced wet strength from anionic filter elements.
The filtration of fine particle size contaminants from fluids has been accomplished by the use of various porous filter media through which the contaminated fluid is passed. To function as a filter, the media must allow the fluid, commonly water, through, while holding back the particulate contaminant. This holding back of the contaminant is accomplished by virtue of the operation, within the porous media, of one or both of two distinctly different filtration mechanisms, namely (1) mechanical straining and (2) electrokinetic particle capture. In mechanical straining, a particle is removed by physical entrapment when it attempts to pass through a pore smaller than itself. In the case of the electrokinetic capture mechanisms, the particle collides with a surface face within the porous filter media and is retained on the surface by short range attractive forces.
With the exception of microporous polymeric membranes, the porous filter media known to the art as being suitable for the filtration of fine particle size contaminants are comprised of fiber-fiber or fiber-particulate mixtures formed dynamically into sheet by vacuum felting from an aqueous slurry and then subsequently drying the finished sheet. In those fibrous filter media that depend upon mechanical straining to hold back particulate contaminants, it is necessary that the pore size of the filter medium be smaller than the particle size of the contaminant to be removed from the fluid. For removal of fine, submicronic contaminant particles by mechanical straining, the filter media need have correspondingly fine pores. Since the pore size of such a sheet is determined predominantly by the size and morphology of the materials used to form the sheet, it is necessary that one or more of the component materials be of a very small size, such as small diameter fibers. See for example, any of Pall U.S. Pat. Nos. 3,158,532; 3,238,056; 3,246,767; 3,353,682 or 3,573,158.
As the size of the contaminants sought to be removed by filtration decreases, especially into the submicron range, the difficulty and expense of providing suitably dimensioned fiber structures for optimum filtration by mechanical straining increases. Accordingly, there is considerable interest in the use of fine particulates such as diatomaceous earth.
However, for such materials it is necessary to provide a matrix in order to present a coherent handleable structure for commerce and industry. Thus, at least one of the component materials in the sheet is a long, self-bonding structural fiber, to give the sheet sufficient structural integrity in both the wet "as formed" and in the final dried condition, to allow handling during processing and suitability for the intended end use. Unrefined cellulose fibers such as wood pulp, cotton, cellulose acetate or rayon are commonly used. These fibers are typically relatively large, with commercially available diameters in the range of six to sixty micrometers. Wood pulp, most often used because of its low relative cost, has fiber diameters ranging from fifteen to twenty-five micrometers, and fiber lengths of about 0.85 to about 6.5 mm.
In addition to controlling the dispersion characteristics (and therefore the porosity of the sheet) and providing wet strength, charge modifiers are employed to control the zeta potential of the sheet constituents and maximize performance in the electrokinetic capture of small charge contaminants. In practice, cationic charge modifiers are employed since most naturally occurring contaminant surfaces are anionic at fluid pH of practical interest. Thus, a melamine-formaldehyde cationic colloid is disclosed for filter sheets in U.S. Pat. Nos. 4,007,113 and 4,007,114.
Biological fluids present a specialized problem in that certain natural substances, commonly of proteinaceous character, are typically present in the system and are preferably, and even preferentially, removed in the course of a filtration operation. Unlike the submicronic impurities more typically encountered in other systems, these materials are cationic in nature at applicable pH values, i.e., below the isoelectric point for such contaminant.
The surfaces of such filter elements as diatomaceous earth, cellulose fiber and the like may also be characterized as weakly anionic in nature, hence it might be expected that these materials would naturally provide the desired electrokinetic properties for enhanced capture potential of the cationic impurities. However, even to take advantage of this somewhat limited effect it is necessary to provide a coherent integral filter medium of controlled, uniform porosity comprised of interengaged filter elements, ordinarily requiring a binding agent for adequate wet strength. Also, higher levels of anionic charge are desired for optimization of the electrokinetic capture mechanism. The use of filter media comprising binders or charge modifiers in filter systems with biological fluids poses special problems, among them the possibility of introducing to the fluid impurities resulting from loss of or a breakdown in filter elements. While certain levels of particular impurities may be tolerable in some systems, organic extractables pose especially sensitive problems in the filtration of foods and pharmaceutical products. In filter systems composed of cellulose fiber as a matrix for particulate filter aids modified with an organic charge modifying resin, organic extractables are naturally primarily traceable to the resin. Selection of the charge modifying resin can alleviate the problem, even under relatively stringent conditions of use including sanitization and sterilizable procedures. Even in the absence of meaningful levels of extractables, however, many resins of choice are subject to discoloration in use, tending to limit their marketability for food and drugs.
Further, even low levels of certain organic extractables are unacceptable in some systems, and accordingly it is desirable for this reason and that of aesthetics to wholly remove the organic charge modifier resin from the filter construction. At the same time, it is desirable for the removal of submicron charged contaminants to retain the charge potential afforded by a charge modifying resin.
The surfaces of the filter elements may be treated with an inorganic charge modifier such as anionic colloidal silica, but by reason of the repulsive effect of these commonly charged materials, only a modest amount of charge modification is effected, and a coherent structure of adequate wet strength may not be conveniently prepared, even at high levels of charge modifier.
U.S. Pat. No. 3,253,978 to C. H. Dexter & Sons Inc. describes a method of preparing a porous, inorganic sheet product of high strength, free of organic binders, composed of inorganic fibers or flakes, e.g., glass or mineral wool bound with colloidal silica in which a cationic agent, e.g., cationic starch, is added to the aqueous slurry containing the anionic colloidal silica binder shortly before deposit upon an inclined Fourdrinier wire. No cellulose containing systems are employed. The patentee compares performance to the similar use of dicyandiamide formaldehyde condensates with cellulose or asbestos in U.S. Pat. No. 3,022,213, evidencing the slow drainage rates experienced. It is probable that the slow drainage rates are a result of the mutual coagulation of the anionic colloidal silica and the cationic starch.
It is accordingly an object of the present invention to provide charge modified filter media sheets of enhanced filtration performance, especially for the removal of submicron contaminants from aqueous systems at high efficiency.
Another object is to provide charge modified filter media characterized by low organic extractables over a wide range of filtration conditions.
A still further object is the provision of filter media effective across the spectrum of biological liquids and, particularly, ingestables such as food and drugs.
A specific object is the provision of anionically charged media of enhanced wet strength from anionic filter elements.
These and other objects are achieved in the practice of the present invention as described hereinafter.